The present invention relates to a method of and an apparatus for processing an image signal, and more particularly to a method of and an apparatus for processing an image signal for sharpness emphasis in an image scanning processing apparatus such as platemaking scanners, facsimile machines, or the like.
Image scanning reading and reproducing systems are widely used in the printing and platemaking industries for electrically processing image information of originals or subjects to produce original film plates with a view to simplifying the entire process and improving the quality of printed images.
The image scanning reading and reproducing systems are basically constructed of an image reading apparatus and an image recording apparatus. In the image reading apparatus, image information of an original or subject which is fed in an auxiliary scanning direction is scanned in a main scanning direction substantially normal to the auxiliary scanning direction, and the scanned image information is converted to an electric signal. Then, the photoelectrically converted image information is processed according to platemaking conditions. Thereafter, the processed image signal is converted to a light signal such as a laser beam signal which is applied to and recorded on an image recording medium of a photosensitive material such as a photographic film in the image recording apparatus. The image recording medium with the image recorded thereon is developed by an image developing device and will be used as a film plate for printing.
If the image on an original is a halftone image such as a photographic image, the image scanning reading and reproducing system effects a sharpness emphasizing process on the image by sharpening the profile of the image for making the image easier to see. The sharpness emphasis is carried out as shown in FIG. 1 of the accompanying drawings, for example. Before an image signal S is processed for sharpness emphasis, (n.times.n) image signals are extracted from around the image signal S, and averaged to generate a local average signal U. Then, a signal S-U indicative of the difference between the image signal S and the local average signal U is determined, and then multiplied by a coefficient signal K. The product is added to the image signal S. As a result, an image signal S which has been processed for sharpness emphasis (hereinafter referred to as a "sharpness emphasized signal") is obtained as defined by: EQU S*=S+K.multidot.(S-U) (1)
A halftone image such as a photographic image is further processed so as to produce a halftone dot image after it has been processed for sharpness emphasis. More specifically, the sharpness emphasized signal S* is converted to a binary signal based on a given halftone dot signal, and a halftone dot image which has been processed for sharpness emphasis is produced on an image recording medium by the binary signal.
The original which bears the image may have a defect produced when the original was formed, a mark or trace of the edge of a photoprint glued thereto, or a deposit of dust or dirt. If the original suffers such a defective condition, then an image signal read by a photosensor is made up of a mixture of a signal component S.sub.O having a larger amplitude and a noise component S.sub.N having a smaller amplitude which is caused by the defective condition, as shown in FIG. 2(a). Consequently, as shown in FIGS. 2(b) and 2(c), the local mean signal U and the differential signal S-U also contain signal components resulting from the noise component S.sub.N. As a result, as shown in FIG. 2(d), the sharpness emphasized signal S* contains a noise component S*.sub.N which has also been processed for sharpness emphasis, as well as a sharpness emphasized signal S*.sub.O resulting from the signal component S.sub.O. When a halftone dot image is formed on an image recording medium based on the sharpness emphasized signal S* which contains the noise component S*.sub.N, an image based on the sharpness emphasized signal S*.sub.O and an image based on the sharpness emphasized noise component S*.sub.N are simultaneously reproduced. The entire combined image thus reproduced is of low quality as the noise component S.sub.N is emphasized.